Yes, skimming over existing wall surfaces, including potentially a previously skimmed wall, is possible, provided the underlying surface is in suitable condition and properly prepared. The success depends heavily on the quality and stability of the existing finish.
Key Considerations for Skimming Existing Walls
Skimming involves applying a thin coat of plaster to create a smooth, paintable surface. When working over an existing finish, whether it's old plaster, render, or a previous skim coat, its condition is paramount.
Based on information regarding skimming over existing plaster finishes, such as lath and plaster walls:
- Existing lath and plaster can be skimmed over.
However, simply applying a new skim coat over an unstable or unprepared surface will likely lead to future problems, such as cracking or delamination.
Conditions for Skimming Existing Surfaces (Based on Reference)
For existing walls or ceilings to be suitable for skimming, the following conditions, highlighted in the context of skimming over lath and plaster, are essential:
- Reasonably Sound Condition: The existing finish must be stable, solid, and firmly attached to the substrate. There should be no major areas of crumbling, looseness, or dampness. Skimming over a deteriorating surface will not fix the underlying issue.
- Adequately Prepared: The surface needs to be cleaned, free from dust, grease, or loose particles. It typically requires a suitable primer or bonding agent to ensure the new skim coat adheres properly.
- Addressing Cracks: Superficial cracks can often be dealt with. The reference notes that "The odd superficial crack is fine provided these are raked out slightly and bridged over with fibre tape (aka scrim)." This technique helps prevent the cracks from reappearing in the new skim coat.
Preparing the Surface
Proper preparation is crucial for a successful skim coat. This generally includes:
- Cleaning the wall thoroughly.
- Repairing any significant damage or large holes.
- Raking out and taping minor cracks using mesh tape (scrim tape).
- Applying a bonding agent or primer suited to the existing surface type.
By ensuring the existing surface meets these criteria, you can significantly improve the chances of achieving a durable and smooth finish when applying a new skim coat.
| Condition | Description | Why it Matters |
|---|---|---|
| Sound Condition | Stable, solid, no significant damage or looseness. | Prevents new skim coat from cracking or failing. |
| Adequate Prep | Clean surface, suitable bonding agent applied. | Ensures proper adhesion of the new plaster. |
| Cracks Addressed | Raked out, taped with fibre tape (scrim). | Bridges cracks to stop them showing through. |
Applying the Skim Coat
Once the existing wall is prepared, the new skim coat (or coats) is applied thinly and smoothed out to create the desired finish.
While the reference specifically mentions lath and plaster, the principles of requiring a sound, prepared base and addressing defects like cracks are universal for skimming over any existing wall finish, including a previous skim coat that may need refreshing or smoothing.
